Shaped pulse DANTE

The double-selective TOCSY-ROESY and TOCSY-NOESY techniques are particularly useful. They allow one to measure NOE and ROE correlations in spectra with high degree of overlap as often found in carbohydrates. In addition to the DANTE, DANTE-Z [66], and Gaussian pulses as described earlier for selective excitation, self-refocusing shaped pulses such as BURP (EBURP and UBURP) [67] have also been used for this purpose [64]. 

DANTE pulse sequences do not require spectrometers equipped with shaped pulses. 

Figure 20 Saturation transfer experiment using (a) Selective inversion by a shaped pulse or (b) a DANTE pulse sequence. Do not use the DANTE-Z sequence, Section 3.3, in saturation transfer experiments...

The excitation sculpting method was expended to the selective inversion of z-magnetisation. The method utilises DANTE train of hard pulses combined with gradient suppression of transverse magnetisation. Application of the selective inversion to z-magnetisation reduces relaxation loses and extends selective methods to larger molecule. Use of shaped pulses in DANTE train produces band-selective inversion. The proposed inversion method can be... 

Fig. iL Pulse sequences for pseudo triple-resonance experiments with passive selection of satellites (a) z-filtered 2D-"X, H( "Y) INEPT. (b) -BIRD H,"X( "Y) HMQC. (c) H,"X( "Y) HMQC experiment with biselective excitation pulse (bis) biselective excitation may be achieved with DANTE or P-BIRD sequences, binomial pulses, or shaped pulses. (d) H, X( "Y) HEED-INEPT." ... 

The major difference between soft shaped pulses and DANTE methods is the occurrence of strong sideband excitation windows either side of the principal window with DANTE. These occur at offsets from the transmitter at multiples of the hard-pulse frequency, 1/x. They arise from magnetisation vectors that are far from resonance and which process full circle during the x period. Since this behaviour is precisely equivalent to no precession, they are excited as if on-resonance. Further sidebands at 2/x, 3/x and so on also occur by virtue of trajectories completing multiple full circles during x. Such multisite excitation can at times be desirable [50,51] but if only a single excitation window is required, the hard pulse repetition frequency must be adjusted by varying x to ensure the sideband excitations do not coincide with other resonances. 

Hard/shaped pulse, soft pulse, DANTE pulse train, composite pulse, cw-irradiation, cp decoupling sequence, spinlock, constant or incremented delay... 

The CSSF element might be considered as an alternative to either selective pulses or a DANTE sequence. However, because of the ease in which shaped pulses can now be implemented, both the CSSF and the DANTE sequence have fallen out of favour. 

The principle of multiple selective excitation has been incorporated into a few ID and 2D experiments, the schemes of which are shown below (fig. 1). Depending on the experiment, either a DANTE pulse train (ID TOCSY [2]), frequency selective 180° pulses (ID NOE [3], ID INADEQUATE [4], ID C/H COSY [5] and 2D TOCSY-COSY [6]) or frequency selective 90° pulses (2D HMBC [11]) are applied to selectively perturb and uniquely label selected spins. Besides the DANTE pulse , composed itself of a series of non-selective rectangular pulses, Gaussian-shaped 180° and... 

Fig. 1. Pulse sequences of ID selective experiments. The shaped 90° pulses were half-Gaussian. The 180° selective pulses were produced by a DANTE-Z pulse train [44]. T denotes the trim pulse of phase ip. tnoe stands for the NOE-mixing time.

Aspects of the experimental implementation of the technique have been studied in detail [41] and it has been shown that double sideband modulation and pulse shaping can be combined to improve the performance of selective pulses in solid state 2H NMR. Applications of the selective inversion-recovery experiment using a DANTE sequence to study ultraslow motions have been demonstrated [42,43]. 

An alternative solution to the problem of selectively observing, for example, the H NMR spectrum of a metal-hydride in protio-, rather than deutero-solvent is to excite just that region of the NMR spectrum that contains signals of interest. This can be done using a shaped soft pulse, using a DANTE (delays alternating with nutation for tailored excitation) type sequence or, on a modem... 

An early alternative to soft pulses was the DANTE Delays Alternating with Nutation for Tailored Excitation) experiment, which used a sequence of short, hard pulses of angle a <3C 90°, followed by a fixed delay t to achieve selective excitation. Thus, the pulse sequence is (a-T), ]. Nuclei that are on resonance are eventually driven to the y axis and hence are selected, whereas those more removed from the frequency range are not affected. The sequence of hard pulses can achieve a result similar to that of soft pulses and even can be shaped by modulating the duration of the pulse lengths, but DANTE pulses lead to spectral artifacts not created by soft pulses, such as unwanted sidebands. 

Shaped and composite pulses are devoid of free precession periods between periods of rf excitation. Pulse sequences combine both free precession and rf pulses. Clearly, given the manifold of shaped and composite pulses, there is an infinite number of pulse sequences which can be constructed from them. A variety of such sequences is used for volume localization. This topic is discussed in Section 10.2. Here two methods relevant to the acquisition of NMR images are reviewed. These are the DANTE technique for single-shot localization, and the Hadamard technique as an example of multi-shot localization. 

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